Glass-lined vessels used in the chemical and pharmaceutical industries often contain large openings, called nozzles, for the passage of contents to and from the vessel. During use, the protective glass coating along the surface of the nozzle often is damaged by sudden impacts and thermal cycling, leaving the underlining metal, which is typically steel, unprotected against the corrosive elements that the vessel may contain.
In glass-lined equipment, any damage to the glass lining, no matter how small, must be immediately repaired in order to prevent corrosion of the base metal and possible loss of the entire equipment. Several types of repair kits are commercially available; one type consists of a metal shield, generally tantalum, a polytetrafluoroethylene (PTFE) gasket, and a filler. Tantalum patch repair procedures include disposing a PTFE sleeve gasket along the damaged surface of the glass liner, followed by the addition of a resinous filler material, and then insertion of the outside tantalum shield over the filler. The tantalum flange is thereafter shaped, or peened, over the outside of the nozzle.
Although tantalum patches have been employed for years, they have some recognized deficiencies. At temperatures above 150.degree. C., for example, tantalum is known to be attacked by concentrated hydrochloric, nitric, and sulfuric acids, bromine-containing chemicals, methanol, and hydrogen gas. It is also subject to galvanic corrosion, and often must be alloyed with tungsten to provide sufficient strength for repairs. Other metals, such as stainless steel, HASTELLOY, MONEL, INCONEL, nickel, titanium, and zirconium have been substituted for tantalum, and these too have similar disadvantages.
In an effort to reduce the labor intensive procedures and attendant costs of metal shields, the art has resorted to PTFE nozzle repair sleeves machined from virgin PTFE resin stock. PTFE nozzle repairs can be made at a fraction of the cost of tantalum repairs, and require no peening or metal working. The PTFE repair shield is simply inserted into the damaged nozzle opening and sealed with a filler material to provide a leak-proof repair. Over the years, PTFE repair shields have been improved with the addition of sealing lips, or fingers, which prevent the seepage of corrosive liquids from attacking the damaged vessel surface. Unfortunately, even these improved shield designs have had some drawbacks. It is known, for example, that PTFE sealing fingers are not very resilient and can heat-set at some of the higher operating temperatures used in chemical processing. If the sealing fingers deform, either during insertion or subsequent use, a small opening in the seal can lead to further damage of the nozzle, or destruction of the vessel. It is also known that, as the diameter of the nozzle repair shield approaches diameters greater than about 30 cm, the PTFE shield loses rigidity, and fails to hold the fingers against the glass surface in a fluid-tight manner. This also permits leakage, which is known to get worse as the temperature of the vessel gets higher.
Accordingly, there is a need for a nozzle repair shield which has greater dimensional stability at chemical processing temperatures, and which will exert a constant biasing force against the nozzle glass surface so as to prohibit leaks beneath the repair.